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I've been having problems with a few statements I've been reading. When setting up DHCP Reservation based on MAC addresses... Do the reserved IP's need to be in one of the scopes or not?

Judging from what I've seen (not sure if this was correct) it had to be in one of the scopes in order to be reserved.

What would be the correct answer to this?

Thanks in advance, Dempsey

  • Windows or Linux? – ewwhite May 14 '13 at 22:00
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According to this definition of scope, if they are not in one of the scopes they would then (implicitly) define a scope - whether that is accepted is up to the server software (which you are not saying) to decide.

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Reserved IP addresses do not need to be within a dynamic IP allocation pool or scope.

This applies to Windows and Linux (ISC) DHCP.

A Linux configuration example where there's a range of .70 through .250, but also static reservations (fixed-addresses) outside of that range at .20, .21 and .22:

subnet 199.3.63.0 netmask 255.255.255.0 {
        authoritative;
        option routers 199.3.63.252;
        option tftp-server-name "199.3.63.3";
        next-server 199.3.63.3;
        filename "/dsl/pxelinux.0";
        range 199.3.63.70 199.3.63.250;
}

host chasea {
        hardware ethernet 00:80:d4:05:20:d6;
        fixed-address 199.3.63.20;
}

host chaseb {
        hardware ethernet 00:80:d4:05:21:0c;
        fixed-address 199.3.63.21;
}

host chasec {
        hardware ethernet 00:80:d4:05:21:0a;
        fixed-address 199.3.63.22;
}
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  • Although, I've been reading that with Server 2008 R2 the address was supposed to be within the scope range to be able to reserve it... is that so? – Dempsey FoxDie Van Assche May 14 '13 at 8:17
  • @DempseyFoxDieVanAssche Are you using Windows? – ewwhite May 14 '13 at 9:46
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Let me rephrase the question as two separate questions. In Windows DHCP:

Q: Does the Reservation have to be in the Scope? A: Yes it does. It has to fall within the subnet defined in the Scope as determined by the subnet mask configured for your Address Pool.

Q: Does the Reservation have to be in the Address Pool defined in the Scope? A: No it doesn't.

You're mixing up terms here. A Scope is more than just the Address Pool defined for the Scope. A Scope also includes Reservations and Scope Options. Think of the Scope as a container. You define the objects in the Scope such as the Address Pool and the Scope Options.

If I have a Scope with an Address Pool defined as 192.168.1.100 to 192.168.1.200 with a subnet mask of 255.255.255.0 then I can create any Reservation that falls withing the same /24 subnet, such as 192.168.1.25.

What I can't do is to create a Reservation that falls within an Address range not defined in my Scope as determined by the subnet mask defined in the Scope, such as 192.168.2.25.

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